1. Field of the Invention
The invention relates to paper machine clothing and more particularly relates to forming fabrics for the fabrication of belts, employed in the forming section of a papermaking machine.
2. Brief Description of the Prior Art
Papermaking machines are well known in the art. The modern papermaking machine is in essence a device for removing water from the paper furnish. The water is removed sequentially in three stages or sections of the machine. In the first or forming section, the furnish is deposited on a moving forming wire and water drained through the wire to leave a paper sheet or web having a solids content of circa 18 to 25 percent by weight. The formed web is carried into a wet press felt section and passed through one or more nip presses on a moving press felt to remove sufficient water to form a sheet having a solids content of 36 to 44 percent by weight. This sheet is transferred to the dryer section of the papermaking machine where dryer felts press the paper sheet to hot steam heated cylinders to obtain a 92 to 93 percent solids content. The efficiency at each state of papermaking is dependent on the efficiency of the preceding step. Thus, overall efficiency is dependent on the effectiveness of the first or forming fabric in the initial step.
Representative of prior art descriptions of prior art forming fabrics are those found in U.S. Pat. Nos. 3,858,623; 4,095,622; 4,149,571; 4,344,464; and 4,453,573.
It is well known in the prior art to produce forming fabric for paper machines wherein the fabric is flat woven and then joined with a seam which has proper papermaking characteristics.
In order to operate successfully on the forming section of the paper machine, a forming fabric must have a given modulus in order to stay within the machine direction length adjustments available on the machine. To achieve the required modulus woven fabrics are heatset under the application of heat and machine direction tension. Depending on the relationship of the diameter and resultant modulus of the machine direction yarns vs the cross machine directions yarns, crimp produced in weaving may be transferred from the machine direction yarns to the cross machine direction yarns. As a result, the machine direction yarns may become essentially straight. Although it is desirable to have the straight machine direction yarn to achieve proper modulus, it is extremely difficult to achieve required seam strength with machine direction yarns that do not have sufficient crimp.
It is the object of this invention to provide a fabric structure which has both sufficient machine direction modulus and proper seam strength to operate successfully.
We have discovered that by proper weaving techniques, it is possible to achieve different crimp configurations in different portions of the machine direction (warp) yarns system. The weave can be arranged in such a manner that a portion of the machine directions yarns are essentially straight, and a second portion of the machine directions yarns have substantial crimp. The first portion will provide the fabric with the proper modulus. The second portion will provide the necessary crimp required to achieve good seam strength.
In order to produce a fabric of this characteristic, at least two independent machine direction (warp) yarn systems must be provided in the loom. This requirement is due to the fact that in weaving the interlacing of the independent warp yarn systems will differ significantly requiring that the warp yarns systems be independently controlled.
With the structured forming fabrics of the present invention, many of the above-described shortcomings of the prior art are removed. Forming belts constructed according to the invention may be fabricated from an all monofilament fabric which is more resistant to degradative elements. The overall operating life of the forming wires is significantly increased over prior art forming wires.